Since the introduction of the CIE (Commission International de l'Eclairage) color measurement system in the early 1930's, many different color spaces have been proposed for different applications. One such color space is the CIE XYZ color space. The CIE XYZ color space characterizes colors by a luminance parameter Y and two color coordinates X and Z which specify the point on the chromaticity diagram. The XYZ parameters are based on the spectral power distribution of the light emitted from a colored object and are factored by sensitivity curves which have been measured for the human eye. The human eye has three different types of color sensitive cones. Accordingly, the XYZ functions were intended to correspond to the average sensitivity of the human eye and provide a device-independent representation of color. Therefore, the spectral responses of the XYZ functions are known as “tristimulus” functions and make up the coordinate system to quantify a color image or color space.
The apparent color of an object depends not only on its intrinsic spectral reflectivity, but also on the spectrum of the light used to illuminate it. The CIE also has defined a number of standard illuminants which are defined theoretically, in terms of their spectral content. To completely specify the color of an object, one must measure the XYZ values of the light emanating from the object when it is illuminated by a standard illuminant.
The process of quantitative color analysis is generally referred to as colorimetry. Currently, most, if not all, precise measurements of object color are made by one of two devices; a tristimulus colorimeter or a reflectance spectrophotometer. A colorimeter uses a light source approximating a standard illuminant and three photoreceptors with spectral responses approximating the XYZ tristimulus responses. The colorimeter is typically a small hand-held device with a circular measurement aperture about 5 mm in diameter. The aperture is held against a colored object and the user initiates a measurement by pressing a button. The measurement typically takes about 1 second. A spectrophotometer is a more sophisticated, flexible and accurate device which measures the entire reflectance spectrum of a colored object using a light source whose spectrum is known. The XYZ values for a user-specified illuminant are calculated from the reflectance and illumination spectra. Spectrophotometers are also typically small hand-held devices and operate similar to calorimeters. However, in many applications the use of calorimeters and/or spectrophotometers is not well suited as these devices suffer from a number of disadvantages. These devices usually require flat object samples and precisely controlled viewing conditions in order to provide a satisfactory measurement of object color. The devices are limited in that they measure the color of a small disk-shaped region one location at a time, and are unable to obtain color measurements for multiple locations on the surface of the object or sample at the same time. The devices are relatively slow. In addition, these devices tend to be expensive due to the manufacturing care necessary to construct a device capable of providing precise color measurements suitable for laboratory use. These disadvantages make these devises particularly unsuitable for the machine vision system environment as they do not have the ability to perform 100% on-line inspection of colorful printed objects (such as floor tiles, credit cards, decorated metal sheets, . . . ) as they are being manufactured.
Color machine vision systems typically utilize a color video camera. A color camera typically generates three separate images: one red (R), one green (G) and one blue (B) referred to collectively as RGB which denotes a coordinate system by which a color can be quantified. These images are typically created by filtering the imaged light with red, green and blue filters. The RGB images are device-dependent values meaning that two different color cameras will typically give two different RGB values for the same object under the same illumination.
Color machine vision systems utilizing color video cameras are capable of 100% on-line inspection, however, to obtain acceptable results when using more than one machine vision system, a standard reference object must be imaged by each system. This can present a problem when using more than one system, especially when the system is in another location. The standard reference object then needs to be shipped back and forth between locations. Reference objects must be carefully stored and handled and can deteriorate with use, especially when sharing the object between two or more distant sites.
Accordingly, there is a need in the art for a color machine vision system that is capable of colorimetry and does not need to share a reference object between two or more color machine vision systems.